Multiple modulating anode beam type electron tube and modulating circuit



Dec. 27, 1966 J. ANDERSON 3,295,066

MULTIPLE MODULATING ANODE BEAM TYPE ELECTRON TUBE AND MODULATING CIRCUIT Filed July 5, 1962 2 Sheets-Sheet 1 H6. 1 if I m I 20 I 42W 1 it 3 K32 I 1 V 1/27 I 1 I I I \x 1 g i 39 40- l i 7 A I 52 Q] |2,/'E |6 l4 k\ K i INVENTOR L J'AMES ANDERSON ATTORNEYS Dec. 27, 1966 J. ANDERSON 3,295,055

MULTIPLE MODULATING ANODE BEAM TYPE ELECTRON TUBE AND MODULATING CIRCUIT Filed July 5, 1962 2 Sheets-Sheet 2 INVENTOR Q IJAMES ANDERSON ATTG RN EYS United States Patent Ofi ice 3,295,066 Patented Dec. 27, 1966 corporation of Texas Filed July 5, 1962, Ser. No. 207,615 14 (llaims. ((ll. 328-232) This invention relates to beam type electron tubes. More particularly, this invention relates to electron tubes such as klystrons wherein means are provided to prevent internal arcing.

Perviously, beam type electron tubes were generally pulse modulated by applying to the cathode a pulse negative with respect to the main body of the tube. The modulator would have to supply the full beam current in addition to the cathode capacitance charging current. For practical reasons, this type of modulation is undesirable because it produces an undesirable phase shift and generates energy over an unnecessarily broad spectrum. It has also been suggested to modulate the beam current, while holding the beam voltage constant, by means of a grid. Such systems avoid the above disadvantages since there are no transit time effects, however, depending upon the grid to cathode spacing there generally results either poor cut-off characteristics or excessive grid dissipation. In addition, the grid disturbs normal focussing, is difiicult to manufacture, and frequently produces a grid heating problem.

The use of a modulating anode has therefore been suggested as a new method to modulate beam type electron devices in either amplitude or pulse modulated applications. A modulating anode is merely the anode of a Pierce type electron gun electrically separated from the tube body, and it is only necessary to switch a small amount of power to charge and discharge the capacitance of the gun and associated circuitry. The cut-off characteristics of the modulating anode are excellent, and in adition, the geometry of the modulating anode gun is the same as that of the Pierce gun so that the normal focussing properties of the gun are retained and the beam shape is not altered during modulation.

Despite the many advantages of the modulating anode there still remains a problem concerning arcing. Generally, the power supply and filter capacitors stored with energy are connected directly from cathode to anode. Should the slightest surface arc, gas burst or interelectrode arc take place, the full energy of the power supply is dissipated into the tube, terminating on the cathode with disastrous results. Various means, electronic and mechanical, have been suggested to prevent application of the excess energy to the tube in the event of an arc. However, such devices have not been satisfactory because of the time it takes for them to operate. Furthermore, if continuous radiation is to be achieved, it is not possible to tolerate a situation wherein the tube is going to are and frequently shut down the power supply.

Accordingly, it is the main object of this invention to materially reduce or prevent high voltage arcing in high power beam type electron tubes.

Another object is to prevent arcing in a klystron without increasing the overall length thereof.

Another object is to provide a multiple modulating tan-ode structure which will provide additional control of electron beam convergence.

Yet another object is to provide a beam type electron tube having increased control of microperveance with respect to electrode spacing, field emission, and unexplained breakdown phenomena.

Still another object is to provide a pulse modulator capable of driving a high impedance electrode with high voltage pulses.

Although the phenomenon of arcing is not quite clearly understood, it has been found that the onset of arcing is dependent on the time and depth of the modulating anode pulse. In accordance with this invention, a plurality of modulating anodes are provided so that the modulating pulse potential may be graded thereacross. In addition, a unique modulating circuit has been designed in which a pair of switching tubes, in response to an input pulse, apply the large operating potentials across the cathode and modulating anodes, and when the input is removed, immediately drop the anode voltages toward the cathode potential.

The manner in which the above objectives are accomplished will be more fully described with reference to the following specification and drawings, wherein:

FIG. 1 illustrates the structure of the gun end of an electron tube according to the invention; and

FIG. 2 illustrates the modulating circuit of the invention.

In FIG. 1, the invention is illustrated for use as the gun end of a klystron, although it is to be understood that the invention would have equal utility with other beam-type electron tubes. The main body structure of the klys-tron, illustrated generally at 10, is conventional and accordingly has only been illustrated in part. The electron beam formed in accordance with the invention passes into the upper portion of the drift tube 12, surrounded by -a conventional space 13 for a circulating coolant, and including a first gap 14. The tube may be a conventional three cavity klystron amplifier, only the first cavity resonator 16 being illustrated.

At the end of the gun, opposite main body 10, a cathode 18 is positioned in front of a'suitable heater element 29 having external heater connections 24 and 26. A tubular focussing electrode 22 surrounds cathode 18 and is operative to focus the electrons emitted from the cathode into a beam.

A ceramic insulating cylinder 27 insulates cathode 18 from the first modulating anode 28. As'illustrated, the modulating anode consists of a flat annular member 30 and an integrally formed hub-like center 31 having an aperture 32 through which the beam from cathode 18 passes. The outer rim of annular member 30 includes a flange 33 for properly positioning the anode. A second ceramic, insulating cylinder 34 separates first modulating anode 28 from a second modulating anode 35. Mod-ulating anode 35 is constructed similarly to anode 28 and comprises fiat annular member 36, hub-like center 37, aperture 38, and flange 39. Aperture 33 maybe smaller than the aperture of 'modulating anode 28 in order to assist in the formation of the beam. The entire gun, as above described, is insulated from the main body structure 10 by means of a third ceramic, insulating cylinder 40.

The klystron is operated by applying a graded modulating potential to both first and second modulating anodes 28 and 35, respectively. The anodes are disposed With respect to the cathode so that the positive potential on the second modulating anode does not affect the cathode. Therefore, when the potential on modulating anode 28 is at the cathode potential, the electron beam will be cut off. As the modulating voltage is increased on the two anodes, the electrostatic field is reinforced to a point at which it affects the cathode surface and the electron current starts to flow. Because very few electrons will land on the modulating anodes, the anodes draw negligible current and consequently consume little or no power.

The main body of the tube functions in the same manner as the radio frequency portion of conventional klystron amplifiers. Radio frequency (RF) driving power is fed into the first resonator 16 and RF output power is taken from the third resonator (not shown), the interaction with the beam involving bunching and debunching of the electrons in accordance with the well known principles of klystron amplifier operation. A magnetic field, pro'duced by conventional means (not shown), may be used to confine the electron beam to a path along the axis of the tube.

It is thought that the increased potential excursion when utilizing a single modulating anode introduces some effect, presently unrecognized, which produces arcing. The modulating anode is generally made of copper, and it has been noted that the copper deposits in a circumferential band on the ceramic insulating cylinders. The fact that the copper deposits in a band suggests that some force is attracting it to the ceramic surface after the arc occurs; possibly the ceramic accumulates a positive charge adjacent to this band. At one hundred kv. (thousand volts) it takes several minutes for this band to accumulate sufficient charge to cause an arc, and since the evidence shows that the arcs are from cathode to modulating anode, the copper band must be charging positively with respect to the cathode.

Because the onset of arcing is related to the increase of modulating anode pulse potential, applicants provision of two or more modulating anodes permits this potential to be graded. For example, the first modulating anode may be driven by a pulse to seventy kv. from cathode, and the second to a voltage of one hundred forty kv. Thus, by the introduction of new electrodes at intermediate potentials, the total voltage per gap is reduced simultaneously with the spacing so that higher electric fields can be sustained in each electrode. Since the electric field strengths tend to remain at their original values when intermediate electrodes are introduced, this results in greater total are stability.

If the modulating anode structures are shortened and made with different diameters, it would be possible to move the modulating anode closer to the cathode in order to obtain greater microperveance and thus permit operation of the tube with less cathode tu-be collector potential. The reduced physical spacing would still result in the same, or less, volts per inch electrostatic stress between the cathode and first modulating anode.

Referring to FIG. 2, a novel modulating circuit is illustrated for applying the necessary voltages to the various electrodes in response to an input pulse. Although representative voltage levels 'will be given, it is to be understood that these values are for explanatory purposes only and are not to be considered in any manner as limiting.

The gun end of the klystron of FIG. 1 is illustrated schematically at the right of FIG. 2. The cathode 18 is connected to a source of high negative potential, e.g., 14O k v., via line 44. The positive side 46 of the high voltage supply is connected to the collector of the klystron, illustrated schematically at 42. The input pulse (which modulates the tube) is applied to the primary of a transformer 45 and may be of a frequency of five megacycles per second and of a desired duration. During the interpulse period, i.e., the period between input pulses, cathode 18 and first modulating anode 28 should be at substantially the same potential (about -140 kv.). The second modulating anode 35 at this time may be maintained at a more positive voltage of about -70 kv. During the pulse period it is desired to drive the first modulating anode 28 positive with respect to the cathode to about 70 kv. and to simultaneously raise the potential on second modulating anode 35 to substantially ground potential, so that the graded potential drop across the two modulating anodes will prevent arcing in the tube. The circuit of FIG. 2 illustrates a novel means for rapidly switching the large voltages employed, and also includes a novel clamping circuit for controlling the positive ex cursion on the two modulating anodes.

In FIG. 2, two separate switching tubes 50 and 52 are provided. Tube 50 includes bias voltage sources 54, 56 and 57. Tube 52 includes two bias voltage sources 58 and 60. During the interpulse period, tube 50 is conducting and tube 52 non-conducting because of the voltages applied by their respective bias sources. With tube 50 conducting, first modulating anode 28 is connected through 'bias voltage source 57 (2 kv.) to the high voltage line 44, so that its voltage is at 142 kv. Because of the voltage drop across resistors 61 and 62, which are connected between the plate of tube 50 and ground, the voltage on the second modulating anode 35 will be at approximately 70 kv. With modulating anode 28 at the cathode potential, the beam current is effectively zero.

When a pulse occurs on the primary of transformer 45, as will be explained in greater detail below, tube 50 is biased to cut-01f and tube 52 starts conducting. Under these conditions, the voltage at modulating anode 28 rises to about -70 kv. while the voltage on second modulating anode 35 rises to about 2 kv. because of the flow of current through resistors 63 and 61 and tube 52. To control the magnitude of the pulse applied to the modulating anodes, a potentiometer 66 is connected across the high voltage line. Potentiometer 66 includes two movable contacts 67 and 68 which are connected through rectifiers 70 and 72, respectively, to the first and second modulating anodes. The voltages on the taps of the potentiometer operate as reference potentials so that when the modulating anode voltages exceed their respective reference levels, the rectifiers are caused to conduct, clamping the modulating anodes to the desired voltage. The operation of this feature will be described in greater detail below.

A pair of triodes 74 and 76 are provided to drive tube 50 to cut-off in the event of an input pulse. Tube "/4 is biased beyond cut-01f because of the positive voltage across resistor 78 produced by bias voltage source 56. Triode 76 is adapted to conduct with a zero grid bias, and is normally conducting during the interpulse period because of bias voltage source 54. When triode 76 is conducting, the positive voltage of source 54 is applied to the grid of tube 50, maintaining this tube in conduction in order to apply the --140 kv. voltage of line '44 plus the 2 kv. voltage of source 57 to modulating anode 28.

If the five megacycle input signal is now applied to the primary winding 77 of transformer 45, a conventional pulse detector 80 develops a sufiicient positive voltage across resistor 82 to drive tube 74 into conduction. Conduction of tube 74 causes the negative voltage of source 56 to be applied to the grid of tube 76 through resistors 84, 86 and 88. This negative voltage cuts off tube 76 and prevents the application of the positive bias voltage from source 54 to tube 50. The removal of this positive bias and the simultaneous application of a large negative bias from source 56 through triode 74 rapidly drives tube 50 to cut-off, and its plate voltage rises to the DC. voltage across the juncture of resistors 63 and 61, which, with tube 52 conducting, will be about 70 kv.

The operation of switching tube 52 is similar to that described above with respect to tube 50. A pair of triodes 90 and 92 are provided to rapidly drive tube 52 from cut-ofif into conduction, bringing second modulating anode 35 to substantially ground potential during the pulse period. During the interpulse period, triode 90 is conducting because of the bias across resistor 94 due to bias source 60. Triode 90 conducts through resistors 95, 96, 97, 98, 99 and 100, the junction of resistors 97 and 98 being connected to the cathode of tube 92, so that because of the voltage drop across resistor 97 and resistors 98 and 99, the grid of 92 is negative with respect to its cathode maintaining triode 92 at cut-off. The cathode of triode 92 is coupled to the grid of switching tube 52, and the voltage drop across resistors 95, 96 and 97 maintains the grid of tube 52 negative with respect to its cathode preventing switching tube 52 from conducting. As explained above, with tube 52 cut oil", the voltage on second modulating anode 35 is the voltage at the junction of resistors 61 and 62 which will be in the neighborhood of 70 kv.

It will be recalled that when a pulse is applied to the primary of transformer 45, tube 50 is rapidly switched to cut-off. The same pulse is simultaneously detected by pulse detector 102 which develops a sufiicient negative voltage across resistor 104 to bias triode 90 to cutoff. With triode 90 cut off the negative bias from source 60 is removed from the grid of triode 92, which conducts under zero bias condition, causing this tube to conduct through resistors 105, 97, 96 and 95, removing the negative bias of source 60 from the grid of tube 52 and simultaneously applying the positive voltage of source 50 thereto. When tube 52 conducts, the voltage at the junction of resistors 61 and 62 rises rapidly toa voltage of approximately 2 kv., or substantially ground.

When the input pulse is no longer present, the circuit returns to its interpulse condition with tube 50 conducting and tube 52 cut-off.

An additional feature of this circuit is the provision of a means for clamping the voltages on the modulating anodes at a desired level. As noted above, a potentiometer 66 is shunted across the high voltage supply. Potentiometer 66 includes two movable contacts 67 and 68 which are connected to rectifiers 70 and 72, respectively. The anode of rectifier 72 is connected through resistors 109, 98, 97 and 96 to second modulating anode 35. When the voltage on anode 35 becomes positive with respect to the reference voltage on tap 68, rectifier 72 passes a current which drives the grid of triode 92 negative to stop conduction through this tube and reduce the positive bias on the grid of switching tube 52'. In this manner, the potential at the second modulating anode is prevented from progressing further toward ground. Similarly, the plate of rectifier 70 is connected to first modulating anode 28 while its cathode is connected to the reference potential at tap 67. Since the voltage on the anode of rectifier 70 cannot be positive with respect to its cathode, by adjusting tap 66 any desired voltage level may be applied to modulating anode 28. The movable contacts 67 and 68 may be mechanically ganged so that any desired ratio of pulse potentials can be applied to the two modulating anodes during the pulse period.

The modulator is capable of handling pulse lengths from a few microseconds to infinity; the pulse repetition rate that can be handled is limited only by the plate dis sipation capabilities of the switching tubes 50 and 52.

Thus, a new structure has been described capable of preventing arcing in beam-type electron tubes employing modulating anodes. Many modifications thereof will be obvious to one skilled in the art and the invention should not be limited except as defined in the following claims.

I claim:

1. In combination, a klystron including an electron emitting cathode and two modulating anodes, a high voltage source, and a pulse modulator, said pulse modulator including means normally connecting the modulating anode closest to said cathode and said cathode across said high voltage source, and means responsive to an input pulse for causing a significant potential difference across said cathode and said one modulating anode and simultaneously increasing the potential between said cathode and the other modulating anode.

2. The combination according to claim 1 including clamping means for limiting the voltage levels on said modulating anodes.

3. The combination according to claim, 2 wherein said clamping means including a potentiometer having two wiper contacts connected across said high voltage source and rectifier means connected between said contacts and said modulating anodes.

4. In combination, a klystron including an electron emitting cathode and at least two modulating anodes, a high voltage source, a higher voltage divider connected between said high voltage source and ground, and a pulse modulator, said pulse modulator including means normally connecting one of said anodes and said cathode across said high voltage source and the remaining anodes to different potential points on said voltage divider, and means responsive to an input pulse for causing a significant potential difference between said cathode and said one modulating anode and simultaneously increasing the potential difference between said cathode and the other modulating anodes.

5. The combination according to claim 4 including clamping means for limiting the voltage levels on said modulating anodes.

6. The combination according to claim 5 wherein said clamping means includes a potentiometer having two Wiper contacts connected across said high voltage source and rectifier means connected between the movable tap of said potentiometer and said modulating anode.

7. The combination according to claim 6 wherein said pulse transformer includes a first control element connected to said high voltage line, first and second bias voltage sources associated with said first element, a second control element connected to a substantially lower voltage, third and fourth bias voltage sources associated with said second control element, switching means connected between said control elements and their respective bias voltage sources so that said first control element is normally conducting and said second control element is normally non-conducting whereby the potential difierence between said cathode and said first modulating anode is substantially zero, and means responsive to an input pulse for operating said switching means so that said first control element becomes non-conducting and said second control element becomes conducting whereby the potential difference between said cathode and said first modulating anode, and said cathode and said second modulating anode is substantially increased.

8. For use with a beam type electron tube including an electron emitting cathode and a modulating anode, a high voltage source, a pulse modulator adapted to connect said high voltage source across said cathode and said modulating anode in response to an input pulse, said pulse modulator comprising a first control element connected to said high voltage line, first and second bias voltage sources associated with said first element, a second control element connected to a substantially lower voltage, third and fourth bias voltage sources associated with said second element, switching means connected between said control elements and their respective bias voltage sources so that said first control element is normally conducting and said second control element is normally non-conducting whereby said modulating anode and cathode are at substantially the same voltage, and means responsive to an input pulse for operating said switching means so that said first control element becomes non-conducting and said second control element becomes conducting whereby a high voltage is applied across said cathode and modulating anode.

9. The combination according to claim 8 including clamping means for limiting the current flow through said second control element to control the magnitude of the voltage drop across the cathode and modulating anode.

10. The combination according to claim 9 wherein said clamping means include a potentiometer connected across said high voltage source and rectifier means connected between the movable tap of said potentiometer and said modulating anode.

11. The combination according to claim 10 wherein said control elements comprise triodes.

12. A pulse modulator circuit for modulating a high voltage source in response to an input pulse, comprising a first control element connected to said high voltage source, first and second bias voltage sources associated with said first element, a second control element connected to a relatively loW voltage, third and fourth bias voltage sources associated with said second element, switching means connected between said control elements and their respective bias voltage source so that said second control element is normally conducting and said first control element is normally non-conducting whereby said output line is connected to said low voltage, and means responsive to an input pulse for operating said switching means so that said second control element becomes nonconducting and said first control element becomes conducting whereby said output line is connected to said high voltage.

13. A modulator circuit according to claim 12 including clamping means for limiting the current flow through said control element to control the voltage level applied to said output line.

14. A modulator circuit according to claim 13 wherein 8 said clamping means includes a potentiometer connected across said high voltage line and rectifier means connected between the movable tap of said potentiometer and said output line.

References Cited by the Examiner UNITED STATES PATENTS 2,434,704 1/1948 Kroger 3327 X 2,842,742 7/ 1958 Preist 332-7 3,051,865 8/1962 Marchese 3153 3,054,015 9/1962 Fujii 315-3.5 3,098,980 7/1963 Dodington 332 7 FOREIGN PATENTS 584,452 1/ 1947 Great Britain.

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

V. LAFRANCHI, Assistant Examiner. 

1. IN COMBINATION, A KLYSTRON INCLUDING AN ELECTRON EMITTING CATHODE AND TWO MODULATING ANODES, A HIGH VOLTAGE SOURCE, AND A PULSE MODULATOR, SAID PULSE MODULATOR INCLUDING MEANS NORMALLY CONNECTING THE MODULATING ANODE CLOSET TO SAID CATHODE AND SAID CATHODE ACROSS SAID HIGH VOLTAGE SOURCE, AND MEANS RESPONSIVE TO AN INPUT PULSE FOR CAUSING A SIGNIFICANT POTENTIAL DIFFERENCE ACROSS SAID CATHODE AND SAID ONE MODULATING ANODE AND SIMULTANEOUSLY INCREASING THE POTENTIAL BETWEEN SAID CATHODE AND THE OTHER MODULATING ANODE. 